Covering materials for electric wires are desired to have a reduced permittivity so as to improve the electrical characteristics. Such reduction in permittivity can effectively be achieved by using foams as covering materials. Foams made of resin material can usually be produced by a foam-molding process in which gas is introduced into a molten resin and then molded.
In order to equalize the shape and the properties of the resulting foam, the cells in the foam are preferably fine and uniformly distributed therein. For the purpose of making the cells fine and of distributing the cells uniformly, a foam nucleating agent, which is to serve as the starting point of generating cells during the foam molding, may be introduced into a resin.
Examples of the foam nucleating agent added to a resin (e.g., fluororesin) include sulfonic acid salts. The following disclosures provide exemplary methods for producing a foam using a sulfonic acid salt.
Patent Literature 1 discloses a foamable thermoplastic resin composition for extrusion molding comprising at least one thermally stable compound as a nucleating agent in an effective amount for nucleation, the compound being selected from the group consisting of sulfonic acids, phosphonic acids, and salts thereof.
Patent Literature 2 discloses a method of foaming a melt-processible fluoropolymer resin in the presence of a boron nitride nucleating agent and (a) at least one polyatomic anion-containing inorganic salt, (b) at least one selected from sulfonic acids, phosphonic acids, and salts of the acids, or (c) combination of (a) and (b).
Patent Literature 3 discloses a foamable thermoplastic resin composition comprising a thermoplastic resin and a nucleating agent system in an essentially effective amount for nucleation, the system comprising (a) at least one selected from sulfonic acids, phosphonic acids, and salts thereof, or (b) at least one polyatomic anion-containing inorganic salt, and the resin composition excluding boron nitride.
Patent Literature documents 4, 5, and 6 disclose a foamable composition comprising a partially crystalline melt-processible perfluoropolymer and a foam nucleating agent. The documents also disclose that the foam nucleating agent contains a nucleating effective amount of at least one thermally stable compound selected from the group consisting of sulfonic acids, phosphonic acids, and salts thereof.
Patent Literature 7 discloses a composition for molding, comprising a melt-moldable tetrafluoroethylene/hexafluoropropylene copolymer, a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, and a foam nucleating agent. The foam nucleating agent is a mixture of boron nitride, calcium tetraborate, and F(CF2)nCH2CH2SO3Ba (n=6, 8, 10, or 12)
Patent Literature documents 8, 9, and 10 disclose combination use of boron nitride having a surface area of 8.6 m2/g with any of sulfonic acids, phosphonic acids, and salts thereof.
Patent Literature 11 discloses a method of extrusion-foaming a melt-processible fluoropolymer resin. In this method, a nucleating agent used is boron nitride, calcium tetraborate, F(CF2)nCH2CH2SO3Ba (wherein n is 6, 8, 10, or 12 in some cases, and mainly 8), or the like.